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+Network Working Group S. Josefsson
+Internet-Draft SJD
+Updates: 4120 (if approved) April 23, 2006
+Expires: October 25, 2006
+
+
+Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges Over
+ TCP
+ draft-josefsson-krb-tcp-expansion-02
+
+Status of this Memo
+
+ By submitting this Internet-Draft, each author represents that any
+ applicable patent or other IPR claims of which he or she is aware
+ have been or will be disclosed, and any of which he or she becomes
+ aware will be disclosed, in accordance with Section 6 of BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF), its areas, and its working groups. Note that
+ other groups may also distribute working documents as Internet-
+ Drafts.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ The list of current Internet-Drafts can be accessed at
+ http://www.ietf.org/ietf/1id-abstracts.txt.
+
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html.
+
+ This Internet-Draft will expire on October 25, 2006.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2006).
+
+Abstract
+
+ This document describes an extensibility mechanism for the Kerberos
+ v5 protocol when used over TCP transports.
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+Josefsson Expires October 25, 2006 [Page 1]
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+Internet-Draft Kerberos V5 TCP extension April 2006
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+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 2. Conventions used in this document . . . . . . . . . . . . . . . 3
+ 3. Extension Mechanism for TCP transport . . . . . . . . . . . . . 3
+ 4. Interoperability Consideration . . . . . . . . . . . . . . . . 4
+ 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
+ 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
+ 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
+ 8. Normative References . . . . . . . . . . . . . . . . . . . . . 5
+ Appendix A. Copying conditions . . . . . . . . . . . . . . . . . . 5
+ Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
+ Intellectual Property and Copyright Statements . . . . . . . . . . 7
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+Josefsson Expires October 25, 2006 [Page 2]
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+Internet-Draft Kerberos V5 TCP extension April 2006
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+1. Introduction
+
+ The Kerberos 5 [3] specification, in section 7.2.2, reserve the high
+ order bit in the initial length field for TCP transport for future
+ expansion. This document update [3] to describe the behaviour when
+ that bit is set. This mechanism is intended for extensions that are
+ specific for the TCP transport.
+
+
+2. Conventions used in this document
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ document are to be interpreted as described in RFC 2119 [1].
+
+
+3. Extension Mechanism for TCP transport
+
+ The reserved high bit of the request length field is used to signal
+ the use of this extension mechanism. When the reserved high bit is
+ set, the remaining 31 bits of the initial 4 octets are interpreted as
+ a bitmap. Each bit in the bitmask can be used to request a
+ particular extension. The 31 bits form the "extension bitmask". It
+ is expected that other documents will describe the details associated
+ with particular bits.
+
+ A 4-octet value with only the high bit set, and thus the extension
+ bitmask all zeros, is called a PROBE. A client may send a probe to
+ find out which extensions a KDC support. A client may also set
+ particular bits in the extension bitmask directly, if it does not
+ need to query the KDC for available extensions before deciding which
+ extension to request.
+
+ If a KDC receive a PROBE, or if a KDC does not support all extensions
+ corresponding to set bits in the extension bitmask, the KDC MUST
+ return 4 octets with the high bit set, and with the remaining bitmask
+ indicate which extensions it supports. The KDC MUST NOT close the
+ connection, and MUST wait for the client to then send a second
+ 4-octet value, with the high bit set and the remaining bits as the
+ bitmask, to request a particular extension. If the second 4-octet
+ value is a PROBE or an unsupported extension, the KDC MUST close the
+ connection. This is used by the client to shutdown a session when
+ the KDC did not support a, by the client, required extension.
+
+ The behaviour when more than one non-high bit is set depends on the
+ particular extension mechanisms. If a requested extension (bit X)
+ does not specify how it interact with another requested extensions
+ (bit Y), the KDC MUST treat the request as a PROBE or unsupported
+
+
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+Josefsson Expires October 25, 2006 [Page 3]
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+Internet-Draft Kerberos V5 TCP extension April 2006
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+ extension, and proceed as above.
+
+ Each extension MUST describe the structure of protocol data beyond
+ the length field, and the behaviour of the client and KDC. If an
+ extension mechanism reserve multiple bits, it MUST describe how they
+ interact.
+
+
+4. Interoperability Consideration
+
+ Implementations with support for TCP that do not claim to conform to
+ RFC 4120 may not handle the high bit correctly. Behaviour may
+ include closing the TCP connection without any response, and logging
+ an error message in the KDC log. When this was written, this problem
+ existed in modern versions of popular implementations.
+ Implementations experiencing trouble getting the expected responses
+ from a server SHOULD assume that it does not support this extension
+ mechanism. Clients MAY remember this semi-permanently, to avoid
+ excessive logging in the server. Care should be taken to avoid
+ unexpected behaviour for the user when the KDC is eventually
+ upgraded. How to handle these backwards compatibility quirks are in
+ general left unspecified.
+
+
+5. Security Considerations
+
+ Because the initial length field is not protected, it is possible for
+ an active attacker (i.e., one that is able to modify traffic between
+ the client and the KDC) to make it appear to the client that the
+ server does not support this extension mechanism. Client and KDC
+ policies can be used to reject connections that does not use any
+ expansion.
+
+
+6. IANA Considerations
+
+ IANA needs to create a new registry for "Kerberos 5 TCP Extensions".
+ The initial contents of this registry should be:
+
+ [[RFC Editor: Replace xxxx below with the number of this RFC.]]
+
+ Bit # Meaning Reference
+ ----- ------- ---------
+ 0..29 AVAILABLE for registration.
+ 30 RESERVED. RFC XXXX
+
+ IANA will register values 0 to 29 after IESG Approval, as defined in
+ BCP 64 [2]. Assigning value 30 requires a Standards Action.
+
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+Josefsson Expires October 25, 2006 [Page 4]
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+7. Acknowledgements
+
+ Thanks to Andrew Bartlett who pointed out that some implementations
+ (MIT Kerberos and Heimdal) did not follow RFC 4120 properly with
+ regards to the high bit, which resulted in an Interoperability
+ Consideration.
+
+ Nicolas Williams and Jeffrey Hutzelman provided comments that
+ improved the document.
+
+8. Normative References
+
+ [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
+ Levels", BCP 14, RFC 2119, March 1997.
+
+ [2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
+ Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
+
+ [3] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
+ Network Authentication Service (V5)", RFC 4120, July 2005.
+
+
+Appendix A. Copying conditions
+
+ Regarding this entire document or any portion of it, the author makes
+ no guarantees and is not responsible for any damage resulting from
+ its use. The author grants irrevocable permission to anyone to use,
+ modify, and distribute it in any way that does not diminish the
+ rights of anyone else to use, modify, and distribute it, provided
+ that redistributed derivative works do not contain misleading author
+ or version information. Derivative works need not be licensed under
+ similar terms.
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+Josefsson Expires October 25, 2006 [Page 5]
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+Internet-Draft Kerberos V5 TCP extension April 2006
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+Author's Address
+
+ Simon Josefsson
+ SJD
+
+ Email: simon@josefsson.org
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+Josefsson Expires October 25, 2006 [Page 6]
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+Internet-Draft Kerberos V5 TCP extension April 2006
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+Intellectual Property Statement
+
+ The IETF takes no position regarding the validity or scope of any
+ Intellectual Property Rights or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
+ this document or the extent to which any license under such rights
+ might or might not be available; nor does it represent that it has
+ made any independent effort to identify any such rights. Information
+ on the procedures with respect to rights in RFC documents can be
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+ Copies of IPR disclosures made to the IETF Secretariat and any
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+ The IETF invites any interested party to bring to its attention any
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+Disclaimer of Validity
+
+ This document and the information contained herein are provided on an
+ "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+ OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+ ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+ INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+
+Copyright Statement
+
+ Copyright (C) The Internet Society (2006). This document is subject
+ to the rights, licenses and restrictions contained in BCP 78, and
+ except as set forth therein, the authors retain all their rights.
+
+
+Acknowledgment
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
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+Josefsson Expires October 25, 2006 [Page 7]
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